Reaction injection molding (RIM) is a molding process for synthetic plastic materials wherein one or more components are injected into a mold under conditions that cause a reaction to quickly occur. The reaction and gellation are completed quickly, and typically demolding is possible between one or two minutes after injection.
Prior RIM systems have employed polyurethane as the major synthetic resin component of the process. However, polyurethane has certain known disadvantages, e.g. high resin cost, poor heat distortion properties, low modulus of elasticity, etc. Also, urethane is formed from a mixture of two approximately equal volume components. Due to poor mixing characteristics of two fibre containing liquids, the potential level of fibre reinforcement is substantially reduced. Urethane components must also be combined in exact stoichiometric proportions, which requires accurate and expensive handling equipment.
Attempts have previously been made to use polyester resin as the main resin constituent in the system to take advantage of its highly desirable molding qualities, e.g. lower cost (approximately 2/3 that of polyurethane), higher heat distortion temperature, high modulus of elasticity, increased reinforcement capability, and increased product potential, i.e. for production of rigid parts. These systems have typically employed nitrogen gas evolved from azo compounds in the injected mixture. However, the nitrogen reaction generates insufficient pressure, i.e. less than the 40 to 50 psi typically required for RIM processes, to resist shrinkage of the polyester, since shrinkage results in unsatisfactory sinks and surface crazing. Further, the system has a short pot life due to assay loss of the foaming agent from the mixture.
It is desirable to provide a RIM system with components that preferably have an extended pot life prior to mixing, that react quickly upon mixing to expand the mixture to fill the mold cavity and cure the resin in the expanded form, and that generate sufficient pressure within the mixture to resist shrinkage. It is desirable that the system further be capable of being substantially reinforced, and produce a completed article that can be demolded soon after injection, e.g. within three minutes.